Copper metal has high electrical conductivity and high heat conductivity, and is widely used as a conductor wiring material, a heat transfer material, a heat exchange material, and a heat dissipating material.
On the other hand, in inkjet printing, a jet dispenser, a needle dispenser, a dispenser, and printing form-based printing, since a liquid material can be applied on a material having an arbitrary shape without using a photoresist process, attention has been paid to these techniques from the viewpoints of on-demand production, power savings, material savings, and cost reduction. Particularly, in inkjet printing and a jet dispenser, where forming is made possible in a non-contact manner, printing can be achieved on steps, curved surfaces or small-area surfaces, and pattern formation that is impossible in printing form-based printing can be achieved.
As a printing ink for forming a copper metal pattern through such printing methods, there have been suggested a dispersion liquid of copper metal nanoparticles (see, for example, Patent Document 1), and a solution or dispersion liquid of a metal complex (see, for example, Patent Document 2). However, copper has a stable oxidation state at room temperature, and copper essentially contains copper atoms in the oxidized state. Therefore, in order for copper metal to exhibit electrical conductivity and heat conductivity, there is a need to reduce those copper atoms in the oxidized state and to prepare a continuum of copper metal.
Furthermore, in a printing ink that uses copper metal nanoparticles, there is a need, if the printing ink contains a dispersant, to first remove the dispersant before use, to subsequently reduce copper oxides, and to sinter/fuse copper metal particles to obtain a continuum. Examples of techniques for such removal of dispersant and/or reduction and sintering include (a) use of hydrogen after activation by an RF plasma method (see, for example, Patent Document 3) or a hot wire method (see, for example, Patent Document 4); (b) flash irradiation of xenon in a hydrogen atmosphere; (c) heating with a polyhydric alcohol having a valence of 3 or higher (see, for example, Patent Document 5); (d) heating in hydrogen gas; and the like.
However, such combinations of printing inks and techniques for reduction and sintering have problems in terms of low adhesiveness, peeling of treated printed layer, high volume resistivity, and deep-part reducibility. Thus, the printing inks could not be applied to conductor wiring materials, heat transfer materials, heat exchange materials, and heat dissipating materials.
The causes for the low adhesiveness, peeling of treated printing layer and high volume resistivity lie in the fact that the printing inks are porous sintered bodies obtained by sintering elemental metal-containing particles in the printing inks through reducing heating, and thereby connecting the particles. During the process of sintering of metal nanoparticles at a temperature much lower than the melting point of the metal, metal atoms migrate within the particles so as to reduce the surface area by utilizing the large surface energy carried by the powder particles and the externally applied energy as the driving force, and bonding/fusion between the particles proceeds (see, for example, Non-Patent Document 1). However, when bonding/fusion between the particles proceeds to a certain extent, and the specific surface area is reduced, the progress of fusion is decelerated and stops. As a result, a sponge-like sintered body is formed. This is because since metal atoms move, after all, only within the powder particles and do not actively precipitate out to the substrate surface, voids remain between the conductor layer and the substrate, and adhesiveness may not be obtained. In order to address such problems, there have been suggested hitherto a method of printing a conductor ink on a precursor of polyimide as a foundation resin (see, for example, Patent Document 6); a method of printing a conductor ink on a semi-cured epoxy resin (Japanese Patent Application No. 2008-267400); and a method of imparting fluidity to a resin that serves as the foundation, making the resin to conform to the conductor layer, and thereby obtaining adhesiveness. However, there are limitations on the foundation resin material or the production method.
For the same reasons, in the process of sintering of metal nanoparticles at a temperature much lower than the melting point of the metal, fusion between the particles stops at a time point at which the specific surface area has decreased to a certain extent along with the progress of sintering, and a porous sponge-like conductor layer is formed. Thereby, there is a problem in a conductorization treatment at or below 200° C. that the volume resistivity does not fall to a value 10 times or less the volume resistivity of bulk copper.
Furthermore, in the techniques of using hydrogen after activation as in item (a), it is reported that the same techniques are effective in the removal of oil films or in the removal of photoresist resins (RF plasma and surface wave plasma: see Patent Document 7, hot wire method atomic hydrogen treatment: see Non-Patent Document 2). As such, in the techniques of using hydrogen after activation, there is also a problem that resin substrates are damaged by activated hydrogen.
Furthermore, according to the investigations made by the inventors, in an activated hydrogen treatment by means of RF plasma or surface wave plasma, deep-part treatability to a depth of 2 μm or more is not obtained, and treatability of deep parts also poses a problem.
On the other hand, as another reduction technique, a reduction technique that uses formic acid gas is known. As a reduction technique that uses formic acid gas, it is reported that a formic acid reflow furnace is effective in the removal of oxide films on the copper and solder surfaces (see, for example, Patent Document 8). This formic acid reflow furnace produces copper formate by applying formic acid gas to copper oxides under heating at a predetermined temperature, and reduces the produced copper formate to produce copper metal. Thus, the formic acid reflow furnace is expected to be also effective in the reduction and metallization of printing inks.